1. Field of the Invention
The present invention relates to duplication processes and devices and more particularly to high-density optical disks and methods and apparatus for the mass duplicating of such data recordings and compact disks for computer, audio and video applications.
2. Description of the Prior Art
Compact discs read only memory (CD-ROM) discs and drives are now ubiquitous in computer data storage, audio recording of musical artists and video. Given the universal demand for CD-ROM titles, the manufacturing of CD-ROM duplicates from masters have assumed very large volumes.
CD-ROM manufacturing is concerned both with duplication fidelity and manufacturing cost. The manufacturing processes used in CD-ROM production closely resemble those used in the fabrication of semiconductors, especially the methods used in mask making.
Mastering, as its name implies, is the process of creating the disc from which all others are produced. The first step in the manufacturing process is to create a master that can be used for replication. A laser can be used to burn pits and lands containing the data into a photoresist surface beginning at the center track and moving outward in a spiral pattern.
After checking a glass master for accuracy, conventional replication machinery makes a stamper. Different replication processes require slightly different stampers, but the function remains the same, embossing the data pattern on the mass-production disc. Through an injection molding process, a series of intermediate impressions are made that provide a generation of negative stampers that produce positive disc images. The family-tree-like structure of this part of the production cycle has given rise to names such as mother, father and sons or daughters for the various disc generations.
Compact discs (CDs) are typically made from a polycarbonate plastic, which is a material that is less vulnerable to water absorption and heat than polymethylmethacrylate (PMMA), which is universally used in laminated videodisks. Videodisks comprise two slices of substrate sandwiched together, so they are more rigid than CDs. Manufacturers take precautions to prevent heat or water-absorption warping, e.g., by using some type of injection molding in which polycarbonate resin is heated and poured into molds that shape the discs. A stamper impresses data patterns into the cooling plastic, and the disc is then put in a vacuum chamber, where a reflective layer of aluminum is added and coated with a protective lacquer. Labels are silk-screened or printed on the lacquer side.
Injection molding has a number of advantages. Plants worldwide use the technique, and its idiosyncrasies are well known. Yields are typically low when a manufacturing plant first opens, and increase substantially as experience is gained. Injection molding's critics claim the process is messy and requires large capital investments in equipment and clean rooms. During molding, polycarbonate distortions can appear in the plastic that impair or deflect a laser reading light. Despite its shortcomings, a number of manufacturing plants operating today use this process.
Minnesota Mining and Manufacturing Company (3M), for example, uses a prior art photo-polymerization (2P) process in which precut polycarbonate precursor resin is inserted between a master and a base plate, and then embossed. This polycarbonate precursor sandwich filling is then cured with ultraviolet light. This replication method has the advantage of being quick, which comes partially by avoiding any heating or cooling of the plastic during production. Critics of this process say yields remain low because improper curing or warpage causes many discs to be rejected.
DOGData of Venlo, The Netherlands, and COMDisc of Los Angeles use two quite different methods that attempt to produce fast, low-cost replication of compact discs by a continuous printing or embossing technique. Both systems have worked in a laboratory setting, but neither is currently available commercially. Although the techniques show promise and have attracted a great deal of attention, no major company has yet committed itself financially to either process.
Masters are original copies of data recordings that are produced from tapes or software provided by artists and programmers. Lasers and electron beams (E-beams) are used as exposure tools for a photoresist carried on glass and photoplates. Semiconductor photomasks are similarly prepared.
Stampers are sub-masters duplicated from masters. Electroplating and photopolymers are two common ways that gaps in resist images are filled to produce reverse-tone sub-master duplicates of the masters so that the ultimate copies manufactured are positives of the masters.
The prior art photo-polymerization (2P) process starts with monomers that are irradiated to form polymers. In data recording disk duplication, such a process requires expensive machinery for ultraviolet irradiation and pressurizing the monomer solutions.
The copending United States Patent Application incorporated herein describes spin coating as a useful technique for reproducing microstructures as small as 0.4 microns and as shallow as 0.1 microns. In a spin-on-and-peel process, a polymer solution is prepared with solvents and purified by filtering. The polymer is first spin-coated onto a master that is to be duplicated. Next, venting and drying of the polymer provides for an exact, but negative replica of the master to be formed as a thin film on the master. The thin-film replica is cured, and then separated by simply peeling it off from the master. Such method is useful for making plastic stampers of plastic to create standard-density CD's, e.g., that store 600 MB. The spin-on-and-peel technique is also suitable for high-density CD manufacturing.
In conventional injection molding, hot molten plastic is injected under high pressure into a containment cavity. Inside the cavity, a template with images in relief, the stamper, is used as a master to transfer its physical image features to the injected plastic. After time, the hot molten plastic cools and hardens to form a solid plastic platter. The features that are copied this way can represent video, audio and/or computer data.
Photo-polymerization is used to make copies in plastic without using added heat. Instead of using hot molten plastic, a liquid photo-sensitive monomer is injected into the cavity containing the plastic stamper. Ultra-violet radiation is used to cure the monomer while in intimate contact with the stamper, the UV radiation solidifies the liquid monomer into a solid to form the plastic platter.
High-density CDs are needed to accommodate the ever-increasing data storage requirements of users. In high-resolution video, the data storage needs demand huge capacities. For a full length movie, a gigabyte or more of memory storage capacity is needed. The design and manufacturing of high-density CDs adequate for such use is presently restricted by the shortcomings of conventional technology which uses standard sixty-ton pressure injection molding and stampers made of nickel. Since high temperatures are used to liquefy the plastic, the stacking of additional image layers over the first image is impossible using hot plastic because the underlayers would remelt. However, an important high-density CD construction technique that uses multiple image layers of pits and lands promises a solution to very high storage density needs. At a minimum, injection molding alone is not a practical way to manufacture high-density CDs.
Various industry consortiums have been formed to cooperate on the development and marketing of high-density CDs. Time, Warner and Toshiba have proposed an alternative two-layer CD structure that can be fabricated by injection molding side "A" then side "B" separately (0.6 mm thick half height) and then joining the two sides back-to-back with aluminum reflective layers in a lamination which yields a 1.2 mm overall thickness. The back-to-back lamination and aluminum reflective layers in between keeps the data images away from the surfaces and thus protects the information from contamination. The two-sided CD is readable with a conventional one-sided CD player, but the diode lasers must be configured to have an unconventional focus length of 0.6 mm, instead of the usual 1.2 mm length. Also it can be read only one side at a time. The second side is accessed by flipping the CD over, as is done with popular vinyl LP records and tape cassettes. Of course, two readers can be combined, one at top and one at bottom, but the equipment would cost more. The method of laminating parts together cannot be used to construct three-layer CDs, because if the over-all thickness is too great, it becomes impossible for the diode laser head to read the deeper layers. Thus to keep the overall thickness within reasonable bounds, multiple layers of thin films would have to be used to make it possible to access the deepest layers. Thin layers are practically impossible to make with injection molding techniques, because it becomes too difficult to maintain a uniform spacing in the cavity and to control warping and shrinkage.
Another CD design consortium, between Sony and Philips, specifies only one reader head on a side, similar to present convention, and uses the standard CD thickness of 1.2 mm. Multiple layers are used that are staged at intervals of 30-40 microns. All the data is accessed from only one side, so the overlying layers must be transparent, or at least translucent. The 3M Corporation has developed a method to manufacture CDs that conforms to the Sony-Philips specification. A photo-polymerization process is used. For example, a layer "A" of a CD is made using standard injection molding techniques. A layer "B" is added to the first layer with a second stamper at the bottom and the first layer at the top with a photo-polymerization monomer layer in between. High pressure is used to force relatively cool liquid monomer into the void between the first layer and the stamper. The pressure is necessary to achieve the intimate material conformance necessary to duplicate the image on the stamper. The injected-in monomer is cured with ultra-violet light, for example, by directing from the top side through the layer "A" plastic. The stamper is conventionally separated from the new CD and sent on for final labeling and printing. The photo-polymerization process introduced the use of monomers and ultra-violet light to cure them, instead of using heating and cooling, as in standard injection molding. Such method is better because more layers can be built-up simply by repeating the same process over and over. Overall thickness can easily be controlled, because each photo-polymerization layer can be a minimum of forty microns thick.
The main advantage of the photo-polymerization process is its usefulness in depositing additional layers of plastics on existing layers of plastics, without disturbing the underlying layers embedded data. But such photo-polymerization process is still an injection method, high pressure is still needed and the making of uniform films precise thickness is difficult. The photo-polymerization process depends on too many variables to control the thickness of each layer very well. Complicated thickness measurement devices could be used to guarantee the desired thirty micron gaps for photo-polymerization material, but that would complicate the process and make the products more expensive.